Many different types of position measuring apparatus are presently known in the art. For example, European Patent Document B1 163362, describes one type of position measuring device in which an apparatus operates by the so-called three-grating measurement principle. The disclosed apparatus has a scale grating which is reflective, and a reference grating with selected characteristics that influence the phase relationship of the various orders of diffraction with respect to one another.
In the dissertation entitled "Photoelektrische Messung der Anderung von Langen-oder Winkelpositionen mit Hilfe von Beugungsgittern" [Photoelectric Measurement of the Variation of Longitudinal or Angular Positions With the Aid of Diffraction Gratings] by F. Hock, University of Stuttgart 1975, the back-mixing of two coherent partial beams on a laminar phase grating is shown (FIG. 55) and described on pages 130 and 131. The measuring grating is not shown, however. The phase grating is embodied such that the zero and even-numbered orders of diffraction are eliminated.
Some devices use polarizer and 1/4 wave plates to create phase displacements of 90.degree. in the detected signals. German Patent Disclosure Document DE-OS 37 00 906 shows a position measuring apparatus operating by interference, in which the phase displacement of the measurement signals are generated by polarization optical elements, which are located in the beam path of the partial beam cluster.
U.K. Patent Application GB 2 185314A discloses a linear and rotary encoder measuring arrangement where the beams incident on the light receiving elements are 90.degree. out of phase with respect to each other through a combination of quarter-wave plates and polarizing plates.
U.S. Pat. No. 4,629,886 discloses an optical scale reader that brings light beams 90.degree. out of phase with each other without the use of polarizers or 1/4 wave plates. Specifically, a half mirror made of plate glass and a semitransparent layer of metal deposited on one surface is used. The diffracted beams are brought into interference and the phase difference of the beams detected from the mirror is determined by the angle of incidence and the angle of the planes of polarization of the incident light beams.